1. Field of the Invention
This invention relates to systems, devices and methods for determining the lithology of a formation while drilling a wellbore. More particularly, this invention relates to systems, devices and methods that utilize dynamic measurements of selected drilling parameters to determine the lithology of a formation being drilled.
2. The Related Art
To obtain hydrocarbons such as oil and gas, boreholes are drilled by rotating a drill bit attached at a drill string end. A large proportion of the current drilling activity involves directional drilling, i.e., drilling deviated and horizontal boreholes, to increase the hydrocarbon production and/or to withdraw additional hydrocarbons from the earth's formations. Modern directional drilling systems generally employ a drill string having a bottomhole assembly (BHA) and a drill bit at end thereof that is rotated by a drill motor (mud motor) and/or the drill string. A number of downhole devices placed in close proximity to the drill bit measure certain downhole operating parameters associated with the drill string. Such devices typically include sensors for measuring downhole temperature and pressure, azimuth and inclination measuring devices and a resistivity measuring device to determine the presence of hydrocarbons and water.
Additional downhole instruments, known as logging-while-drilling (“LWD”) tools, are frequently attached to the drill string to determine the formation geology and formation fluid conditions during the drilling operations. Logging-while-drilling (LWD) systems, or more generally speaking, Measurement-While-Drilling (MWD) systems, are known for identifying and evaluating rock formations and monitoring the trajectory of the borehole in real time. An MWD set of tools is generally located in the lower portion of the drill string near the bit. The tools are either housed in a section of drill collar or formed so as to be compatible with the drill collar. It is desirable to provide information of the formation as close to the drill bit as is feasible. Several methods for evaluating the formation using the drill bit have been employed. These methods eliminate the time lag between the time the bit penetrates the formation and the time the MWD tool senses that area of the formation. However, they work only under certain formation characteristics. Another approach to determine formation or lithology changes has been to use the mechanic measurements available at the surface, such as measured rate of penetration (ROP) and bit revolutions per minute (RPM) and average or mean downhole weight on bit (WOB) and average or mean downhole torque on the bit (TOR) that are derived from real time in situ measurements made by an MWD tool.
While conventional devices and methods for determining lithology have provided adequate results only in very few applications, there is a persistent need for systems, devices and methods that provide more accurate indications of the lithology being drilled.